Patent battles are won and lost each day, but a fresh defeat for Bristol-Myers Squibb (BMY) may prompt greater scrutiny of so-called composition of matter patents, which essentially refers to the mixture of two or more substances. In what appears to have taken some analysts by surprise, a federal court judge yesterday decided that the Bristol-Myers patent on the Baraclude hepatitis B treatment was invalid.

Why? The judge ruled that this particular patent is obvious, which is a way of saying the invention would have been obvious to someone ‘skilled in the art’ of designing and developing drugs. In other words, there was nothing so unique about the composition of matter patent for Baraclude that Bristol-Myers could cite this as a reason for fending off generic rivals which, in this instance, is Teva Pharmaceuticals (TEVA).

The potential significance, however, is not the specific fate surrounding Baraclude, which generated $1.4 billion worldwide sales last year. Rather, the ruling appears to raise questions about “the robustness of composition of matter patents on products with only minor modifications from older drugs,” Sanford Bernstein analyst Ronny Gal writes in a research note. “…The case has the potential to reverse well-accepted drug dogma.”

Bristol-Myers had been optimistic enough about the outcome of this case to have built Baraclude revenue into its forecasts. But given that the drug has been in decline and the patent expires in 2015, the sudden potential for diminished sales is not expected to require the drugmaker to revise its forecast, ISI Group analyst Mark Schoenebaum writes in a note. He estimated Baraclude would generate about $260 million in US sales this year.

Ed Silverman, a contributing editor of YCharts, is the founder and editor of Pharmalot. He previously reported on the pharmaceutical industry and other business topics for the Star-Ledger of New Jersey, New York Newsday and Investor’s Business Daily. He can be reached at editor@ycharts.com.